在我的GWT项目上获得null结果

Question

我正在使用Eclipse + GWT来调用iTunes API。这是我想获取信息的URI：

  

itunes.apple.com/es/rss/topsongs/limit=25/json

控制台没有给我任何错误，也没有给我任何错误。 这是我认为使ChartSearch为null的代码的一部分： （每个Pastebin链接都是Java类）

public class ITunesAPI implements EntryPoint {
    private Button searchButton = new Button("Search");
    private TextBox searchField = new TextBox();
    private HorizontalPanel searchPanel = new HorizontalPanel();
    private Label statusLabel = new Label();


    private final iTunesServiceAsync iTunesService = GWT.create(iTunesService.class);

    public void onModuleLoad() {
            searchField.setText("País");

            searchPanel.add(searchField);
            searchPanel.add(searchButton);
            searchPanel.add(statusLabel);

            // Add panel to the page
            RootPanel.get("form").add(searchPanel);

            // Focus the cursor on the name field when the app loads
            searchField.setFocus(true);
            searchField.selectAll();


            searchButton.addClickHandler(new ClickHandler() {

                    public void onClick(ClickEvent event) {

                            statusLabel.setText("Searching...");

                            final String pais = searchField.getText();
                        RootPanel.get("iTunes").clear();

                        try{
                            iTunesService.getChart(pais, new AsyncCallback<ChartSearch>(){

                                    public void onSuccess(ChartSearch result) {
                                            statusLabel.setText("Exito");
                                            showChart(pais, result);

                                    }


                                    public void onFailure(Throwable caught) {
                                            statusLabel.setText("Error");

                                    }
                            });
                        }catch(Exception e){
                            statusLabel.setText("Error (try/catch): " + e.getMessage());

                        }
    }

            });
    }


    private void showChart(String pais, ChartSearch result) {
            String output="<fieldset>";
            output += "<legend>" + pais + " Top 25 Chart</legend>";
    if (result != null) {
            for (Entry a : result.getFeed().getEntry()) {
                    output +="<span>" + a.getTitle().getLabel() + "</span><br/>";
            }
    }else
            output="<span>No results</span>";

    output +="</fieldset>";

    HTML albums = new HTML(output);

    RootPanel.get("iTunes").add(albums);
    }


public class iTunesServiceImpl extends RemoteServiceServlet implements iTunesService{
    String country;

    public ChartSearch getChart(String country) {
            this.country = country;
            ClientResource cr = new ClientResource("https://itunes.apple.com/" + country + "/rss/topsongs/limit=25/json");
            ChartSearch message = cr.get(ChartSearch.class);
            return message;
    }

如果您需要项目的其余部分或某个特定课程，请告诉我，我很乐意上传它。

谢谢，